Automatic frequency corrected transmission system



Dec. 11, 1951 J H, HAMMOND, JR 2,578,541

AUTOMATIC FREQUENCY CORRECTED TRANSMISSION SYSTEM Filed Feb. 9, 1949 3 Sheets-Sheet 1 NI? l5 POWER PRE- AMPLIFIER g2 23 l AMPLI F l ER 1 (28 I A CONTROL 7' I n I! TERS SHUTTERS as E1 OPEN l SHUTTERS AVERAGE 3 0 "4 SHUTTERS AVERAGE q. 9 o .05 E s 'TP-5 u: D e O FREQUENCY FREQUENCY Snventor JOHN HAYS HAMMOND, JR.

Gnome Dec. 11, 1951 J HAMMOND, JR 2,578,541

AUTOMATIC FREQUENCY CORRECTED TRANSMISSION SYSTEM Filed Feb. 9, 1949 3 Sheets-Sheet 2 Sh ong lnpu'l' 3. f 6 Q- 3 Zhwentor Fr quency JOHN HAYS HAMMOND, JR.

Dec. 11,

Filed Feb.

J. H. HAMMONDLJR 3 Sheets-Sheet 3 001' put Vbi'l'aga Frequency (I I4 IL IOQ n5 Ihmemor Gttornwp Patented Dec. 11, 1951 OFFICE AUTOMATIC FREQUENCY CORRECTED TRANSMISSION SYSTEM John Hays Hammond, J r., Gloucester, Mass.

Application February 9, 1949, Serial No. 75,403

1 This invention relates to a sound pickup system for recording or broadcasting purposes, from pipe organs in, which the tonal volume is controlled by swell shutters interposed between the chambers in which the pipes are located and the auditorium in which the pickup microphone is placed.

It has been observed that the operation of such swell shutters is more effective in varying the transmission of the higher frequency components of the soundwaves, and that the re-' sulting records which are musically correct with the shutters open, do not have a sufliciently high ratio of high frequency to low frequency components when the shutters are closed.

This invention provides for automatic variation of the transfer characteristics from the microphone to the recording head or line as the general sound level at the microphone is varied. More specifically it provides that the ratio of high frequency transfer to low frequency transfer shall be greater for low volume than for high volume.

The invention also consists in certain new and original features of construction and combinations of parts hereinafter set forth and claimed.

The nature of the invention as to its objects and advantages, the mode of its operation and the manner of its organization, may be better understood by referring to the following description, taken in connection with the accompanying drawings forming a part thereof, in which Fig. 1 is a schematic diagram of one form of the invention in which the transfer characteristics are controlled mechanically;

Fig. 3 is a series of curves illustrating the variation in gain characteristics,

Fig. 3 is a series of curves illustrating the corresponding currents in the recorder head windings;

Fig. 4 is a. schematic diagram showing the circuits for an electronic method of automatic control of thetransfer characteristics;

Fig. 5 is a series of curves illustrating the output of the circuits shown in Fig. 4;

Fig. 6 is a schematic diagram showing the 8 Claims. (Cl. 84-1-27) they are intended to be generic in their application to similar parts.

Referring to the drawings and more particularly to Fig. 1, a swell pedal I0 is pivoted at H and provides for opening swell shutters, not shown. to give maximum opening when. the pedal Ill is moved forward (clockwise in Fig. l) to engage stop I2 and to provide closure when the pedal I0 is moved backward to engage stop 13. The sound from the organ pipes, also not shown, passes through the swell shutters and energizes a microphone l4, which is connected through a preamplifier [5, variable network l3, power amplifier I! and line I8 to a recording head [9 which cuts a record driven by turntable 2| in the usual manner. The power amplifier Il may be provided with manual volume and tone controls indicated at 22 and 23.

In more detail, the variable network I6 is 20 provided with input terminals 24, 25 and output terminals 26, 21, the former being connected to the output of the preamplifier l5 and the latter to the input of the power amplifier H. The terminals 25 and 21 are connected to a grounded 25 line 28. The terminal 24 is connected through a resistor 29, a resistor 33, paralleled by a capacitor 3|, and through a capacitor 32 and a rheostat 33' to the ground line 23. The elements 32 and 33 constitute a tone control arm of the network I6 which is paralleled by a potentiometer 34 in series with a resistor 35, which constitutes a volume control arm of the nework ii.

The movable contact 36 of rheostat 33 is rigidly supported from a movable bar 33, of insulating material, at point 39; the movable contact 31 is rigidly supported from the bar 33 at point 43. The contact 36 is connected to the point 33.

thence through conductor 4|, rigidly mounted on the bar 38, and flexible take of! conductor 43 to ground line 28. The contact 31 is connected to the point 40, thence through conductor 43 and flexible take off conductor 44 to the output terminal 26. The bar 38 is movable vertically. through guides not shown. To provide motion of the bar 38, the upper end is connected through a cable 45, a coil spring 48 and a cable 41 to the lower end of the pedal l0, and the lower end of the bar 38 is connected through a cable 43 to the upper end of the pedal Ill. The cable 45 passes over suitable pulleys 49. 53, and ii and the cable 48 passes over a suitable pulley 52. These pulleys are suitably located so that rotary motion of the pedal H) in a clockwise direction. as indicated by the arrow, causes the bar 43 to move up vertically, as indicated by the arrow.

of the swell pedal and swell shutters.

The spring 46 provides a take up to keep the cables taut. It is thus seen that the pedal l and the control bar 38 are mechanically linked, so that motion of the pedal It) to open the swell shutters decreases the amount of resistance of rheostat 33 between the lower terminal of capacitor 32 and ground, thereby making the capacitor 32 more effective in reducing the amount of high frequency voltage across potentiometer 34 and resistor 35 in series. Therefore the motions of the pedal Hi to open the swell shutters increases the proportion of total voltage across potentiometer 34 and resistor 35, in series, that is delivered to the output terminal 26. The opposite effect is produced if the pedal l0 and consequently the bar 38 are moved in the opposite sense corresponding to closing the swell shutters. By making the impedance of rheostat 33 sufliciently great, it is seen that the elements 32, 33 can be made to produce no discrimination against high frequencies when the shutters are closed, butvery considerable discrimination against high' fre-' quencies when the swell shutters are opened. There is of course some degree of discrimination at an average setting of the pedal l3, due to the shunt circuit 32, 33 ofthe network, but this is compensated for by use of the capacitor 3| across the resistor 30 in the feed line, which enhances the higher frequency voltage delivered to the elements 32, 33. The use of the variable potentiometer 34 provides for compensating for the lowered average electrical level which would otherwise result as the bar 38 moves to diminish the transmission of high frequencies. If it is desired to permit a slight amount of overall com- .pression, the compensating feature by use of potentiometer 34 may be omitted. If on the other hand, a considerable degree of compression is desired, the potentiometer 34 may be used with reversed connections to capacitor 32 and resistor 35, so that motion of the pedal l6 and bar 33 as indicated by the arrows, will decrease the amount of resistance between movable contact 31 and ground through the elements 34, 35. It

. purposes of illustration.

In Fig. 2, the operation of the circuit for the connection shown is given in terms of the ratioof output voltage e: to input voltage ei as a function of frequency and as a function of the setting This shows that the transmission of highs relatively to lows is increased as the swell shutters are closed, but

in such a manner that the transfer, averaged over the frequency range is fairly independent of the shutter position, and in such a manner that the transfer is fairly uniform when the shutters .are in the average position.

It will be understood, of course, that the network |6 may include amplifying elements, and may be varied as to tone and volume characteristics not by mechanical connection with the pedal, but by the amount of signal that is impressed upon the microphone circuit.

A circuit of this type is shown in Fig. 4 which provides for relatively lowered transmission of high frequencies at high signal volumes, and conversely increased relative transmission of high frequencies at low signal volume, in combination with an overall compression, so that the recording will automatically compensate for the excessive dynamic range provided. This arrangement responds, of course, to the general tone level at the microphone, regardless of whether it is varied by operation of the swell pedal, or by the choices of organ stops.

The circuit shown in Fig. 4 may take the place of the network l6 of Fig. 1 and is according y Provided with similarly numbered input terminals 24,25 which will'be understood to be connected output terminals 26, 21 which will be understood rent to the recording head, taking into consideramain effect, of course, is the compensation by network |6 for the observed fact that high frequencies are not sufilciently recorded when the swell shutters are closed relative to the recording of the low frequencies.

to be connected to an electrical circuit such as a power amplifier.

Across the input terminals 24 and 25 is connected a potentiometer 53, the variable contact 54 of which is connected through a capacitor 55 to contact 56 of a single pole double throw switch 51. The terminal 25 is grounded. The input terminal 24 is connected through resistor 58 and capacitor 59 in parallel and capacitor 60 to the rid 6| of the first section of a duo-triode tube 62. The anodes of both triodes are connected in parallel and through a feed resistor 63 to the positive end of a battery 64 the negative end of which is connected to ground. The anodes of the duo triode 62 are also connected through a capacitor 65, of negligible impedance, to one end of each of two potentiometers 66 and 61, the

other ends of which are connected to ground.

The terminal 21 is also connected to ground. The variable contact 68 of potentiometer 66 is connected to output terminal 26, while the variable contact 69 of potentiometer 61 is connected to the contact 10 of switch 51. The junction of capacitor with the potentiometers 68 and 63 is connected through two capacitors 1| and 12 in series to the grid 13 of the second section of the duo-triode 62, and the junction of capacitors 1| and 12 is connected to ground through a resistor 14. The constants of the circuit are so chosen that signals of substantially all frequencies are impressed from, the input terminals 24, 25 upon the grid 6|, with the circuit 56, 59 increasing the transmission of high frequencies. The constants are also so chosen that high frequency energy is transmitted back from the common anodes of the duo-triode 62 to the grid 13 of the second section, through the high pass filter comprising elements 1|, 12, 14 whereby there is a high frequency voltage on grid 13 substantially out of phase with the corresponding high frequency part of the voltage on grid 6|. tends to reduce the overall amplification of the system by an amount depending upon the conductances and internal impedances of the two sections.

For varying the transmission characteristics of the system, the cathodes of the two sections of tube 62 are connected together and through two resistors 15 and 16 in series to ground. Two capacitors 11 and 16 are connected across the resistors 15 and 16 respectively. Grid 6| is con- This nected to the junction of resistors 15 and I8 through a resistor I9, from which it receives its signal voltage, and a resistor 80, shunted by a capacitor 8|, from which it receives a variable control voltage due to curent flowing in the resistor 80 in the direction shown by the arrow. The grid I3 is connected to ground through a resistor 82, from which it receives its signal voltage, and a resistor 83, shunted by a capacitor '84, from which it receives a variable control voltage due to current flowing in resistor 88 in the direction shown by the arrow. When the currents in the resistors 80 and 83 are negligible, it is thus seen that the second section of tube 62 is more highly biased than the first section, asfor example with the first section operating as a class A" amplifier, and the second section biased at cutoff with little or no space current flowing.

To provide variable currents in the resistors 80 and 83, the movable blade 85 of the switch 51 is connected through contact I to the variable contact 69 of the potentiometer 61 which is energized from the anodes of the tube 82. The blade 85 of switch 51 is also connected to ground through a resistor 88, and to the grid 81 of an amplifier triode tube 88, the cathode of which is connected .to ground through a resistor 89 paralleled by a capacitor 90. The anode of tube 88 is connected through a feed resistor 9| to the positive end of a battery 82, the negative end of which is connected to ground. The anode of tube 88 is also connected through a capacitor 93 to the cathode of a rectifier 94 which in turn is connected through a resistor 95 to the junction of resistors 80, I and I8. The anode of the rectifier 94 is connected through two resistors 86 and 91 to the junction of the resistors I8 and 80, and the junction of the resistors 08 and 81 is connected to the junction of the resistors 80 and I5, I6 through a capacitor 88. The anode of tube 88 is also connected through a capacitor 80 to the anode of a rectifier I00 which in turn is connected to ground through a resistor IOI. The cathode of rectifier I00 is connected through two resistors I02 and I03 to the junction of the resistors 82 and 83, and a capacitor I04 is connected from the junction of the resistors I02 and I03 to ground.

The triode 88 and associated circuits described constitute a rectifier system operated from the output of the tube 62 to develop unidirectional currents in the resistors 80 and 83 in a well known manner to vary the operating conditions of the two triode sections of the tube 62. Thus increased output from the tube 82 drives the grid 8| more negative, thereby reducing the overall gain of the system, and at the same time drives the grid I3 more positive thereby increasing the feedback and decreasing the high frequency response of the circuit relative to the low frequency response. As a result, the system behaves in general as a compressor, but with the greater amount of compression for the higher frequencies. The control of the. compression is by adjustment of the potentiometer 61, and the amount of output is controlled by the adjustment of potentiometer 88. Since the output is a decreasing function of the input voltage, it is possible to obtain a greater degree of compression by transferring the blade 85 of switch 51 to contact 56, so that the rectifier circuit is energized from the input potentiometer 53. It will be understood that for comparable performance, the tube 88 should be fed with about the same amount of signal at mean level from either potentiometer 53 or 61. It will be understood that the circuit as a whole will be adjusted so that there is negligible tonal distortion, so that the rectifier circuit is more responsive to the medium frequencies than to the very low frequencies, and so that the sum of the two anode currents of the two sections of tube 82 is substantially independent of the signal volume.

Although in Fig. 4 a high pass feed back network has been shown comprising elements II, 12, I4, it will be understood that for different purposes a different type of feedback may be desirable. Thus for example, if it is desired for some reason that both high and low frequencies be strengthened at low signal level, relative to medium frequencies, then a feedback network may be substituted which produces degenerative effects both for low and high frequencies and regenerative effects for mean frequencies.

Fig. 5 illustrates the response of the circuit shown in Fig. 4 to composite tones with various frequencies present. When the composite tone is weak, the grid I3 produces negligible effect, but the highs are emphasized due to the treble boost elements 58, 59. As the composite tone grows stronger, the feedback to the grid 13 produces a degenerative effect at high frequencies, thereby lessening the discrimination between low and high frequency components. Thus at low signal levels, the high frequency components are emphasized with respect to the low frequency components. The situation is similar to the conditions the circuit shown in Fig. 1 except that the operation is automatic in accordance with the input to the circuit, and independent of the reasons for the variation in signal level. Thus the circuit provides for preemphasis of the high frequencies at low volume, which tonally may be of value in minimizing needle scratch disturbances although not necessarily musically correct.

A circuit for emphasizing the high frequency response at low volume is shown in Fig. 6, which provides for a slight overall expansion to counteract the compression on high frequencies. Thus the output signal level may be proportional to the input signal level, although the relative intensities of low and high frequencies change. This circuit also may be substituted for network I8 of Fig. l, and is provided with like numbered input terminals 24 and 25, and output terminals 26 and 21, similarly connected to a preamplifier and a power amplifier.

In this circuit, a potentiometer I05 is connected across the input terminals 24 and 25, the latter being grounded. The terminal 24 is connected to one end of a capacitor I08 of negligible impedance, the other end of which is connected through a resistor I01 tothe grid I08 of one section of a duo-triode tube III and through a capacitor I00 to the grid IIO ofa second section of this tube. The anodes of the tube III are connected together, and through a feed resistor I I2 to the positive end of a battery I I3, the negative end of which is connected to ground. The anodes are also connected through a capacitor H4, of negligible impedance, and a resistor I I5 and an output potentiometer H8 to ground. The potenti ometer I I8 may be shunted by a capacitor I I1, and-- the variable potentiometer contact H8 is connected to output terminal 20. The terminal 21 is connected to ground. For this circuit, the grid I08 is energized by all frequencies, while the grid H0 is energized by the higher frequency component substantially in phase with energization of 7 the grid I08. Therefore the high frequencies are especially emphasized when the conductance of the second section is high. The output circuit including elements H5, H6, I" may be chosen to counteract the high frequency emphasis if desired.

To provide variations in the tonal distribution, the cathodes of the tube I II are connected together and through resistor I I9, paralleled by capacitor I20, to ground. The grid I08 is connected to ground through a resistor I2I, from which it receives its signal voltage, and through a resistor I22, shunted by a capacitor I23, from which it receives its control voltage due to the current in the resistor I22 flowing in the direction indicated by the arrow. The grid H is connected through a resistor I24, from which it receives its signal voltage, and through a resistor I25 shunted by a capacitor I28, from which it receives its control voltage due to the current in resistor I25 flowing in the direction indicated by the arrow, to ground. The constants of the circuit are such that when the control currents are zero, both triodes which 'are of like characteristics, are biased somewhat greater than for class A operation. When the control currents are a maximum, the bias in the first section is reduced so that it operates substantially as class A. While the bias on the second section is increased so that it approaches a cutoff condition, so that the high frequency preemphasis due to the second section is made negligible. It will be understood that the bias changes on the two sections may be of different value by suitable choice of constants, and that the circuit may be 30 arranged that the sum of the D. C. components of the anode current is substantially constant.

To vary the control, the tap I21 of potentiometer I is connected to one side of a capacitor I28,,the other side of which is connected through aresistor I29 to ground and through a resistor I30 to the grid I 3I of the first section of a duotriode tube I32. The cathode of this first section is connected to ground through a resistor I33, the anode is connected to ground through a resistor I34, and is connected through a resistor I35 to the positive end of battery I36, the negative end of which is connected to ground. The anode of the first section is also connected through a capacitor I3'I to the anode and the grid of the second section of tube I32, and these in turn are connected through a resistor I38 and resistor I39 to the junction of the resistors I24 and I25. Capacitor I40 is connected from the junction of the resistors I38 and I39 to ground. The cathode of the second section of the tube I32 is connected through a resistor I and a resistor I42 to the junction of resistors I2I and I22, and a capacitor I43 is connected from the junction of resistors HI and I42 to ground. This circuit is seen, therefore, to provide an amplifier and rectifier by which the amounts of current in resistors I22 and I25 are, for weak and medium signals, proportional to the acoustical value of the input to the system. The constants ofthe resistors associated with the gridand anode circuits of the first or amplifier section of tube I32 may be so chosen that grid and plate limiting action may set in at strong signals vto prevent excessive changes of bias on the two sections of tube III. Moreover, the impedances of the elements I28, I29, I30 may be sufiflciently high with respect to that of the potentiometer I05 that negligible tonal distortion results whenever grid limiting action takes place.

In this circuit, the expansion due to the control upon the grid I08 of tube III applies to all frequencies, while the compression due to the control upon the grid IIO applies to the higher frequencies only. By proper choice of constants, the high frequency compression effect by grid no may be made about twice the high frequency expansion effect by grid I08, so that the net result is about the same amount of high frequency compression as there is low frequency expansion. In this case the average tonal output may be proportional to the average tonal input.

Fig. 7 shows pictorially the relative output strength of low and high frequency components of a composite tone of varying input level. This performance is comparable with that of Fig. 3, with the action of the grid I08 analogous to that of the contact 31, and the action of the grid III analogous to that of the contact 36.

Although only a few of the various forms in which this invention may be embodied have been shown herein, it is to be understood that the invention is not limited to any specific construction but might be embodied in various forms Xithout departing from the spirit of the inven- What is claimed is:

1. A system fortranslating the sound output of a musical instrument having mechanical volume control means and manual actuating mechanism therefor, comprising a sound pickup microphone disposed to respond to the sound from said instrument as affected by said volume control means, a translating device responsive to electrical energy from said microphone, and a transmission channel interconnecting said microphone and said translating device, said transmission channel including an electrical network having variable frequency transmission characteristics, and means controlled by said manual actuating mechanism to vary the frequency transmission characteristics of said network as a function of the volume control introduced by said mechanical volume control means.

2. A system for translating sound energy as set forth in claim 1 in which said network is caused to discriminate more in favor of the higher frequencies as the volume control is shifted to reduced volume position.

3. A system for translating sound energy as set forth in claim 1 in which said network discriminates in favor of high frequencies for increased volume setting of said manual means and in favor of the lower frequencies for decreased volume setting of said manual means.

4. A system for translating sound energy as set forth in claim 1 in which said mechanical volume control comprises organ swell shutters actuated by said manual means.

5. A system for translating the sound output of a musical instrument having mechanical volume control means and manual actuating mechanism therefor, comprising a sound pickup microphone disposed to respond to the sound from said instrument as affected by said volume control means, a translating device responsive to electrical energy from said microphone, and a transmission channel interconnecting said microphone and said translating device, said transmission channel including an electrical network having variable frequency transmission characteristics, and means including a triode connected to provide a negative feedback having a control grid connected through a high pass feedback network to the output end of said cban- *9 nel to thereby reduce the relative ion characteristics having high frequencies as a function of the transmission level of said network.

6. A system for translating the sound output of a musical instrument having mechanical volume control means and manual actuating mechanism therefor, comprising a sound pick up microphone disposed to respond to the sound from said instrument as affected by said volume control means, a translating device responsive to electrical energy from said microphone, and a transmission channel interconnecting said microphone and said translating device, said transmission channel including an electrical network having variable frequency transmission characteristics, and means to alter the frequency transmission characteristics of said network as a function of signal volume, said last means comprising a pair of triodes having anodes and cathodes connected in parallel and having control grids connected to automatic biasing resistors, and means supplyingbiasing voltages to said resistors proportional to the signal volume and in opposite sense so as to vary oppositely the conductivity of said triodes.

7. An electronic single stage amplifier for the output of a microphone, comprising a pair of triodes having anodes and cathodes connected in parallel, and having a common anode output circuit, one triode having a grid connected to be actuated by the microphone signal and the second triodehaving a grid connected to be actuated from the high frequency portion of the signal in said common anode output circuit, and means varying the bias on the grid of the first triode as an increasing function and the bias on the grid of the second triode as a decreasing function of the amount of signal input to said triodes so as to provide overall compression of the signal level and greater compression of the high frequency components of the signal.

8. A system for recording music from organs and the like, comprising a microphone responsive to said music, a recorder to record said music as -a record and a dynamic amplifier interconnecting said microphone and recorder, said amplifier having means responsive to the transmission level and connected to alter the frequency transmission characteristics of said amplifier to produce stronger recording of high frequency components of the signal relative to that of low frequency components for lower general input levels than for higher input levels.

' JOHN HAYS HAMMOND. JR.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

